Chromatography separates components of a mixture based on their differential affinity between a stationary phase and a mobile phase.
Stationary Phase
The fixed medium (paper, silica on a TLC plate). Polar substances interact more strongly with polar stationary phases → travel less far.
Mobile Phase
The solvent that moves through the stationary phase. Components dissolve in and travel with the mobile phase at different rates.
Retention Factor (Rf)
Retention Factor (Rf)
\[ R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}} \]
Aspirin Purity & Rf Calculation
A TLC sample is run. The solvent front travels 6.0 cm from the baseline, and a visible spot travels 3.0 cm.
\[ R_f = \frac{3.0\text{ cm}}{6.0\text{ cm}} = 0.50 \]
Because Rf is a ratio of two distances measured in identical units, the units cancel out: Rf is always dimensionless (between 0 and 1).
Common Chromatography Errors
- Rf values are dimensionless (no units) and must be between 0 and 1
- Measure from the centre of the spot, not the top or bottom edge
- A single spot → pure substance. Multiple spots → mixture/impurities
- If Rf = 0, the component is insoluble in the mobile phase - try a different solvent
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